It is a modern trend of the vehicles to employ a power transmission device for vehicles which automatically executes the shifting operation or the clutch operation for easy driving. A representative example is a so-called automatic transmission (AT) combining a torque converter and a planetary gear type transmission together, i.e., a power transmission device which uses a transmission (MT) of the type of parallel shaft gear mechanism which is the same as that of the so-called manual transmission vehicle in combination with an automatic clutch. In recent years, a power transmission device has been developed for use in vehicles equipped with a diesel engine by interposing a fluid coupling between the engine and the automatic clutch. Upon interposing the fluid coupling, it is made possible to attain smooth start by utilizing the slipping between the pump and the turbine in the fluid coupling at the start of the vehicle and, at the same time, absorbing fluctuation in the engine torque during idling and reducing vibration and noise.
The engine of the power transmission device having the above-mentioned automatic clutch includes an engine control device for determining the fuel feed amount (fuel injection amount) relying upon the amount the accelerator pedal (numeral 62 in FIG. 4) is depressed by a driver and the engine rotational speed. When normally traveling, the fuel injection amount control (accelerator pedal follow-up control) is executed by using the amount the accelerator pedal is depressed as a basic parameter. At the time of gearshift by disengaging the clutch for the shifting operation, the engine control device switches to a control mode for controlling the engine independently of the amount the accelerator pedal is depressed (gearshift engine control) to meet a sudden decrease in the engine load as a result of disengaging the clutch, and reverts to the accelerator pedal follow-up control at a moment when the clutch is engaged. To execute the above controls, the engine control device works in cooperation with a transmission control device that controls the transmission and a clutch control device that controls the clutch. The control mode at the time of gearshift is changed over not being limited in the power transmission device having a fluid coupling interposed therein but also in the power transmission devices for vehicles in general.
FIG. 4 schematically illustrates a conventional power transmission device for a vehicle equipped with a fluid coupling and an automatic clutch. In the power transmission device, a fluid coupling 2 is coupled to the rear of an engine (diesel engine) 1, and a transmission 4 having a parallel shaft gear mechanism is coupled thereto via a clutch (wet multiple disk clutch) 3. An output shaft 41 of the transmission 4 drives the wheels 7 of the vehicle. The fluid coupling 2 works at the start of the vehicle, and includes a lockup clutch 23 for fastening a pump 21 (integral with the output shaft of the engine 1) and a turbine 22 (integral with an input shaft 32 of the clutch 3) together. After the start of the vehicle, when the gear is to be shifted in the transmission 4, the lockup clutch 23 fastens the pump 21 and the turbine 22 together and, therefore, the output shaft of the engine 1 is directly coupled to the input shaft 32 of the clutch 3. The transmission 4 is an ordinary transmission of the type of parallel shaft gear mechanism in which a gear spline integrally formed in the gear meshes with a shift sleeve, and includes a known synchronizing mechanism comprising a transmission input shaft (integral with the output shaft 33 of the clutch 3) and a synchronizer ring. The above power transmission device for vehicles has been disclosed in Japanese Patent No. 3724491 assigned to the present applicant.
At the time of gearshift of the vehicle, the shifting operation is effected by disconnecting the transmission of power from the engine 1 to the transmission 4 by using the clutch 3. Here, the amount the clutch is engaged is controlled depending upon a duty ratio D(%) of the pulse output from the clutch control device 31. Namely, the clutch 3 is completely engaged when the duty ratio is 0% and the amount of engagement becomes zero when the duty ratio is 100% so as to be disengaged. An engine control device 11 provided for the engine 1 controls the engine 1 in cooperation with a clutch control device 31 that controls the amount the clutch 3 is engaged and a shift control device 61 that controls the operation of the transmission 4. To execute these controls, there are arranged a clutch input shaft rotational sensor 51, a transmission input shaft rotational speed sensor 52 and a transmission output shaft rotational sensor 53.
FIG. 5 is a timing diagram illustrating operation characteristics of the control devices at the time of shift-up of the power transmission device of FIG. 4. In FIG. 5, if the vehicle that is traveling at, for example, the second speed is accelerated to increase its speed, the shift control device 61 produces a shift instruction A so as to be shifted to a suitably shifted state, i.e., so as to be shifted to the third speed. Responsive to the shift instruction A that is output, the clutch control device 31 outputs the duty ratio of 100% to disengage the clutch 3 (the actual amount of engagement becomes zero being slightly delayed behind as shown). At a moment the clutch 3 is disengaged, the shift control device 61 disengages the gear of the second speed, places the transmission 4 in the neutral state, and brings the gear into engagement with a gear of the third speed through the synchronizing action by the synchronizing mechanism. The reduction ratio of the transmission is smaller at the third speed than at the second speed. Therefore, in a step where the gear is shifted from the second speed to the third speed by disengaging the clutch 3, the rotational speed of the input shaft of the transmission 4 suddenly decreases down to a rotational speed corresponding to the third speed. At the time of shift-down from the third speed to the second speed during the deceleration of the vehicle, on the other hand, the rotational speed of the input shaft of the transmission 4 suddenly increases.
Responsive to the shifting instruction A that is output, the engine control device 11 changes the control mode from the accelerator pedal follow-up control over to the gearshift engine control, and decreases the amount of fuel injection to meet a sudden decrease in the engine load as a result of disengaging the clutch to prevent the engine rotational speed from sharply increasing. In controlling the engine at the time of gearshift, the amount of fuel injection is, first, decreased stepwise down to the amount of fuel nearly at the time of idling (graduation control) and, thereafter, the amount of fuel is so controlled that the engine rotational speed approaches the rotational speed of the transmission input shaft at the third speed. At the time of shift-down from the third speed to the second speed, the engine rotational speed is controlled to remain in the present state or to increase by a predetermined amount.
At a moment when the gears of the transmission 4 are engaged, the clutch control device 3 starts controlling the clutch 3 so as to be engaged. In controlling the clutch 3 so as to be engaged, the duty ratio is sharply decreased down to about 0% in a short period of time (called “one-shot engagement”) and, thereafter, the so-called half-engaged clutch control is executed to return the duty ratio back to the state where the clutch is half-engaged and to gradually decrease the duty ratio again. The duty ratio is set to be nearly 0% for a short period of time from the standpoint of stabilizing the operation of the clutch piston of the clutch (wet multiple disk clutch) 3 to attain the state where the clutch is half-engaged in an early time. The actual amount of engaging the clutch is as shown; i.e., the amount of engagement with which the clutch starts half-engaging is quickly reached and in the state where the clutch is half-engaged, the amount of engagement mildly increases nearly along a change in the duty ratio. This avoids the shift shock and engine stall at the time of engaging the clutch.
As the amount of engagement of the clutch increases in the state where the clutch is half-engaged, the engine rotational speed further decreases, the rotational speed of the transmission input shaft increases, and the two rotational speeds gradually get close to each other. At a moment (B) where the difference has reached a predetermined value, the control mode of the engine control device returns from the engine control at the time of gearshift back to the accelerator pedal follow-up control, and a control operation is executed using the amount the accelerator pedal is depressed by the driver as a basic parameter. At the time of returning back to the accelerator pedal follow-up control, too, the graduation control is executed to increase the amount of fuel injection stepwise.
Patent document 1: Japanese Patent No. 3724491